1. Technical Field
The present disclosure generally relates to a method and associated instrumentation for implant insertion and, in particular, to a method and instrumentation for insertion of a spinal implant to facilitate fusion of adjacent vertebral bodies.
2. Background of the Related Art
A large number of orthopedic procedures involve the insertion of either natural or prosthetic implants into bone or associated tissues. These procedures include, for example, ligament repair, joint repair or replacement, non-union fractures, facial reconstruction, spinal stabilization and spinal fusion. In a typical procedure, an insert, dowel or screw is inserted into a prepared bore formed in the bone or tissues to facilitate repair and healing. See, for example, U.S. Pat. Nos.: U.S. Pat. No. 5,470,334 to Ross et al.; U.S. Pat. No. 5,454,811 to Huebner; U.S. Pat. No. 5,480,403 to Lee et al.; U.S. Pat. No. 5,358,511 to Gatturna et al.; and U.S. Pat. No. 4,877,020 to Vich.
Some implants are particularly configured with cavities and bores to facilitate bony ingrowth and enhance anchoring of the implant at the insertion site. See, for example, U.S. Pat. No. 4,328,593 to Sutter et al.; U.S. Pat. No. 4,936,851 to Fox et al.; and U.S. Pat. No. 4,878,915 to Brantigan. Other specialized implants include fusion cages having internal cavities to receive bone growth stimulation materials such as bone chips and fragments. See, for example, U.S. Pat. Nos.: U.S. Pat. No. 4,501,269 to Bagby; U.S. Pat. No. 4,961,740 to Ray et al.; U.S. Pat. No. 5,015,247 to Michelson; and U.S. Pat. No. 5,489,307 to Kuslich et al. These types of implants are particularly well suited for intervertebral spinal fusion procedures necessitated by injury, disease or some degenerative disorder of the spinal disc. Subsequently, there may be progressive degeneration leading to mechanical instability between adjacent vertebrae necessitating direct fusion of the vertebrae while maintaining a pre-defined intervertebral space. This fusion may be accomplished by the insertion of one or more of the specialized implants as discussed above and also discussed in commonly assigned U.S. Pat. No. 5,026,373, the contents of which are incorporated herein by reference.
Both anterior (transabdominal) and posterior surgical approaches are used for interbody fusions of the lumbar spine. Fusions in the cervical area of the spine are primarily performed using posterior and approaches as well. Typically, an implant such as a plug, dowel, prosthesis or cage is inserted into a preformed cavity inside the interbody, interdiscal space. Since it is desirable in these procedures to promote a "bone to bone" bridge, connective tissue and at least a portion of the distal tissue is removed. Preferably, relatively deep cuts are made in the adjacent bones in order to penetrate into the softer, more vascularized cancellous region to facilitate bone growth across the implant.
One of the more critical tasks performed in the insertion of a surgical fusion implant, particularly, in intervertebral spinal fusion, is the formation of the implant receiving cavity or bore within the adjacent vertebrae. More particularly, the drilled bore must be equally centered within the intervertebral space and preferably parallel to the vertebral end plates to ensure removal of equal portions of bone from the adjacent vertebrae throughout the length of the cut and subsequent appropriate seating of the implant relative to the vertebral bodies.
Surgical instruments for facilitating spinal fusion implant insertion are known. For example, U.S. Pat. No. 5,484,437 to Michelson discloses a method and apparatus incorporating an outer and an inner sleeve arrangement. The outer sleeve has teeth at one end which are driven directly into the posterior surface of the adjacent vertebrae. The inner sleeve is positioned within the outer sleeve and serves to guide instruments such as a drill used to form the implant receiving bore. U.S. Pat. No. 5,487,307 to Kuslich et al.; U.S. Pat. No. 5,015,247 to Michelson; and U.S. Pat. No. 4,878,915 to Brantigan disclose similar arrangements. Other arrangements include the use of guide rods which are placed in pilot holes formed in the vertebral bodies. The guide rods guide a bore forming hollow drill into the intervertebral space.
Although current instrumentation and methods associated therewith for enhancing the placement of spinal fusion implants have been generally effective for their intended purposes, there exists certain limitations with the design of this instrumentation which detract from their usefulness. For example, the arrangement disclosed in the Michelson '437 patent and similar arrangements do not provide for automatic alignment of the outer sleeve to ensure that the bore formed by a drill introduced into the outer sleeve is in optimal alignment for a tapping procedure (if required) and reception of the spinal implant. Rather, such orientation is dependent directly upon the skill of the surgeon. Moreover, the outer sleeve, which is mounted only at its extreme distal end to the posterior surface of the adjacent vertebrae, is subject to disorientation or dislodgment during insertion and/or removal of the drill and/or tapping instrument. Similarly, the use of guide rods increases the number of steps required to implant the fusion cage and is also subject to possible misalignment.
Commonly assigned U.S. patent application Ser. No. 08/889,661, filed Jul. 8, 1997, which claims priority to Ser. No. 08/615,379, filed Mar. 14, 1996, the contents of which are incorporated herein by reference, discloses a unique method and instrumentation for effecting fusion of adjacent vertebral bodies. The method and instrumentation incorporates a retractor, commercially known as the USSC Tang Retractor, which performs several functions upon its placement between adjacent bodies. Specifically, the USSC Tang Retractor is advantageously configured to: 1) distract the adjacent vertebrae a desired predetermined special distance to facilitate the insertion and application of an implant therethrough; 2) provide a cannula for reception of other instrumentation needed to perform the procedure; and 3) ensure accurate alignment of the instruments and accurate insertion of the implant.